Posted on Nov 23, 2012

Oscilloscope measurements of ground noise can be unreliable because noise can enter your circuit via the scope`s three-pronged power plug. You can avoid this problem by using the ground-noise tester shown. Powered by two 9-V batteries, the circuit dissipates power only while push-to-test switch Sl is depressed. Noise pulses that reach IC2A`s switching threshold of about 1.5 to 1.8 V create a logic transition that triggers the monostable multivibrator IC3, which stretches the pulse to produce a visible blink from LED Dl.

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You set the noise reference level by adjusting threshold-adjust potentiometer Rl, which lets the circuit respond to minimum pulse amplitudes ranging from about 0 to 1 V. For convenience, you can use a onetum potentiometer for Rl and calibrate the dial by applying an adjustable de voltage, monitored by an accurate voltmeter.

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